Position of third charged particle

1. The problem statement, all variables and given/known data
Two particles are separated by a distance d, Particle A has charge +Q and Particle B has charge +3Q. At what distance from particle A along the line connecting particles A and B would you place a third charged particle such that no net electrostatic force acts on it ?

2. Relevant equations
F = kq1q2 / r^2

3. The attempt at a solution
Since there is no net electrostatic force on the third particle, the force from Particle A to C, and force from Particle B to C must be equal.

Note that they don't define d in the statement of the problem, yet they state their answer in terms of d. That's a clue right there that there's something amiss. Perhaps a typo!

I also agree with .366, but d is given.

The author seems to have made two errors:
Taken distance as inversely proportional to square of force, instead of the other way around;
Taken the ratio of the two distances instead of the ratio of the shorter distance to the total distance.
Very sloppy.